The present invention relates to diffusion of impurities into a semiconductor body and more particularly to diffusion of impurities reactive with the diffusion tube into a semiconductor body.
Typical conventional diffusion techniques include open tube diffusion and sealed or closed tube diffusion. According to the open tube diffusion, a carrier gas is allowed to flow into a diffusion tube from one end and exhausted through the other end. Diffusion source vapor is mixed into the carrier gas at some location either outside or inside of the diffusion tube. On the other hand, sealed tube diffusion utilizes a sealed tube containing a diffusion source and semiconductor wafers. The diffusion tube is evacuated with or without the backfill of an inert gas before sealing. Naturally, there is no way of exhausting or ventilating the diffusion chamber after sealing. Moreover, after the diffusion process at a high temperature, the diffusion tube is cut for unloading the diffused semiconductor wafers. The diffusion tube, usually made of quartz, is discarded after each diffusion run.
There is also proposed evacuated open tube diffusion which may resemble the sealed tube diffusion in the point that the diffusion chamber is evacuated. In contrast to the sealed tube duffusion, the diffusion tube is continuously evacuated and the impurity vapor is freely exhausted.
U.S. Pat. No. 3,997,379 to Rosnowski proposed evacuated open tube method wherein vacuum drawn during diffusion is regulated as an additional control of the diffusion.
Rosnowski also proposed semi-closed open tube high vacuum diffusion of aluminum into silicon, in J. Electrochem. Soc.: Solid-State Science and Technology, June 1978, pp. 957-962, wherein a "restrictor" is inserted into the open end of a quartz diffusion tube for regulating the aluminum vapor outflow rate during the process. The diffusion tube is pre-treated at 1100.degree. C. to have a coating of mixture of alumina and silicon, which serves as a passivation layer during diffusion.
There is a general problem in diffusing those impurities which react with the diffusion tube. The reacted diffusion tube may be devitrified and become weak in strength. Coating of a passivation material as employed by Rosnowski may serve to reduce the reaction itself, but requires an extra step in diffusion process.
Decrease in the strength of a diffusion tube is dangerous, particularly when the diffusion tube is to be evacuated. It is desired that the diffusion system can be repetitively used safely.